1. Field of the Invention
The invention relates to sealing arrangements which are used for sealing axially movable rods or pistons, in order to seal a high-pressure side from a low-pressure side.
2. Description of Related Art
For these seals, frequently sealing edge rings are used, which are made of tough, elastic plastic based on PTFE or a harder thermoplastic which have one or more sealing edges at their inner circumferential surface. On their outer side, the sealing edge rings have a profile ring made of a softer elastomer material additionally pressed on radially and sealed. Sealing edge ring and profile ring are installed in a housing groove. When several seals are arranged one after another on a rod, in the case of translatory motion of the two machine parts to each other, an inverted pressure situation can occur, whereby the pressure difference reverses itself, that is, the low-pressure side becomes the high-pressure side, and the high-pressure side becomes the low-pressure side. This may cause inadmissible deformations to appear on the sealing edge ring, which may result in leakage or early failure of the sealing arrangement. Pressure reversal occurs particularly when a pressure drop takes place on the high-pressure side, and the pressure build-up in the sealing space between the primary and secondary seal remains at a higher pressure level, or drops to the level of the high-pressure side only with a great delay in time. In this connection, the pressure in the sealing space can clearly be above the maximum operating pressure. Besides the rise in frictional force in the entire sealing system, under these circumstances the sealing edge ring of the primary seal may be twisted. The initial contact compression of the sealing edge is interfered with by plastic deformation and wear to such an extent that a sufficient sealing function is no longer present.
From DE 196 54 357 A1 a sealing arrangement has become known in which the contact surface of the sealing ring is provided with at least one connecting duct via which the high-pressure side and the low-pressure side may be connected. Clamping ring and sealing ring having the connecting duct are aligned with each other in such a way that the high-pressure side is closed off from the low-pressure side by the clamping ring and that, in an inverted pressure situation, the connecting duct is freed. In this respect the clamping ring works like a check valve.
Another solution to the problem may be seen in DE 36 20 539 A1. There, also, a sealing ring made of elastic plastic and a press-on ring are used. On its inner surface facing towards the rod, the sealing ring has two axially displaced annular sealing edges, and has at least one duct starting from the inner surface, running between the sealing edges and opening out into the outer surface of the sealing ring. The opening of the duct is covered by the clamping ring. In this manner there is created a sort of safety valve, and, as soon as the intermediate pressure between the sealing edges exceeds the value of the surface compression in response to which the clamping ring lies on top of the orifice of the duct, the clamping ring lifts off from the opening, and fluid can flow back again into the space to be sealed via the duct. Using this solution, the space formed by two sealing edges of a seal is ventilated.